1. Field of the Invention
This invention relates to the general field of film characterization and, in particular, to a technique for improving the process of identifying the boundary and thickness of a thin film by interferometric profilometry.
2. Description of the Prior Art
It is well known that light is reflected at the interface between two media with different indices of refraction. By interfering the light reflected from the interface of the two media with a reference beam in a scanning interferometer, an interferogram is generated with maximum contrast at a scanning position corresponding to the interface. Thus, the precise location of the interface along the scanning direction may be identified by determining the peak of the modulation envelope of the interferometric output beam generated by the scan. As is well understood in the art, this may be achieved using one of several techniques as may be most appropriate depending on the wavelength of the light, the bandwidth of the light, the data acquisition scheme, and other factors relevant to the particular interferometric system in use.
When a transparent film is present in a structure, its thickness produces two interfaces with respective reflections of an incident beam. If both reflected beams are interfered with a reference beam in a scanning interferometer, the resulting interferogram will contain two peaks of maximum contrast, each corresponding to the location of the interface between the film and the abutting material. Based on the scanning position corresponding to each contrast peak, the relative optical-length distance between interfaces may be calculated in conventional manner. The film thickness is then derived directly by dividing the optical distance by the group index of refraction of the film material.
Based on these well-known principles, it has been a practice to calculate the thickness of a film by acquiring interferometric light-intensity data during a scan to produce an interferogram from which the two peaks corresponding to the film's interfaces are identified. A light of appropriate bandwidth is used to ensure sufficient coherence length to produce interference at the scanning positions corresponding to both sides of the film and to generate identifiably separate modulation envelopes. Accordingly, a light-intensity threshold is typically used to separate intensity data corresponding to interferometric fringes from noise and constant (DC) signal components.
According to the most general prior-art approach, illustrated in FIG. 1, only data above or below (or both) predetermined thresholds T′ and T″ around the DC component are used to isolate noise and to identify the intensity measurements corresponding to the two regions of significant contrast R1, R2. Modulation envelopes are then derived from these data and used to find the peaks P1,P2 of the regions and the corresponding scanning positions Z1,Z2. Finally, the difference between the peak positions (Z2-Z1) is divided by the group index of refraction to obtain the film thickness at the location corresponding to the pixel associated with the acquired interferogram. FIG. 2 shows interference fringes corresponding to the intensity data of FIG. 1 which illustrate the progressively higher contrast seen in the vicinity of the peaks P1, P2.
In practice, it is more convenient to work with modulation data, rather than intensities. In its simplest form, modulation may be defined as the absolute value of the difference in light intensity recorded between two consecutive data-acquisition frames during a scan (alternatively, the square of the intensity difference is also used). Accordingly, as illustrated in FIGS. 3 and 4 for a single correlogram with peak P1, the modulation M (FIG. 4) of the output of the interferometer (FIG. 3) may be calculated at each scanning step and compared to a first threshold value T to eliminate data that do not reflect the presence of fringes. Since modulation increases materially at scanning positions corresponding to the vicinity of an interface (as seen in FIG. 2), the threshold T is judiciously selected to eliminate data that correspond to noise.
While this approach is theoretically sound and relatively easy to implement, in practice it is often difficult to set the threshold T at the appropriate level. If it is set too high, it becomes difficult to identify peaks characterized by relatively low maximum contrast (such as P2 in FIG. 1). If it is set too low, regions of particularly great noise may be mistakenly identified as regions of contrast and the analysis produces erroneous film-thickness results. This balance is particularly critical when measuring very thin films because the separation between the interfaces of the film sides and the adjacent media may not be sufficient to provide distinct contrast regions. That is, the signals produced by the two interfaces may overlap and yield artifacts that mask the true modulation data produced at each interface. In addition, even when the modulation envelopes are conveniently separated and the threshold parameter is set to clearly distinguish the regions of contrast from noise, different threshold parameters may be needed for materials with different indices of refraction or scattering properties in the film. Therefore, it is typically not possible to utilize this prior-art interferometric approach to characterize multiple film layers.
The present invention is based on the realization that the shortcomings of the prior-art approach may be overcome by relatively simple modifications that produce an optimal threshold level regardless of the film material and system noise. As a result, this disclosure also enables the characterization of single- as well as multi-layer film structures.